Process for preparing medium density granular sodium tripolyphosphate

ABSTRACT

Finely divided sodium tripolyphosphate (STPP) particles are sprayed with a solution of sodium orthophosphate to prepare an agglomerated feed material which is then calcined at 300° to 600°C to produce granular STPP of medium range bulk density.

The present invention relates to a method of preparing granular sodium tripolyphosphate (STPP) having a controlled bulk density. More particularly, this invention relates to a method for treating finely divided STPP particles by an agglomeration technique which, after calcining, results in granules having a bulk density within a desired range of 0.50 to 0.80 grams per cubic centimeter (g./cc.), and improved particle strength or a low degree of frangibility.

Sodium tripolyphosphate (STPP) is commonly prepared by first reacting phosphoric acid and an alkali compound such as sodium hydroxide or sodium carbonate in an aqueous solution such that the mole ratio of sodium to phosphorus is about 1.67:1. This reaction results in a formation of an aqueous mixture of monosodium orthophosphate and disodium orthophosphate in a mole ratio of about 1:2. This mixture is then heated at about 300°C. and STPP is formed therefrom. The reaction proceeds as follows: NaH₂ PO₄ + 2Na₂ HPO₄ → Na₅ P₃ O₁₀ + 2H₂ O.

The resulting STPP is a crystalline anhydrous product capable of having two physical forms. Form I is produced at temperatures of from about 450°C to 600°C. and Form II at temperatures below about 450°C.

The heating of the orthophosphate feed solution is conventionally carried in either a spray dryer or a rotary kiln. Spray dried products generally have a bulk density of about 0.45 to 0.65 g./cc. Bulk density is the weight of STPP which freely flows into a container of given volume. A convenient method for measuring bulk density is the Solvay Process Method 302A described in the Solvay Technical and Engineering Service Bulletin No. 9 (page 33) issued in 1944.

When STPP is prepared in a rotary kiln, the resulting granular product generally has a relatively high bulk density, that is, in the order of about 0.90 to 1.00 g./cc. Thus, the conventional preparative techniques do not allow for the production of granular STPP in appreciable amounts having a bulk density in the medium range, that is, about 0.55 to 0.75 g./cc. Materials in this range are particularly desirable for the preparation of dry mixed detergent formulations. The present invention provides a process for the agglomeration of finely divided STPP particles in order to form an agglomerated feed which, upon calcining, has a bulk density within the desired range.

Techniques for preparing STPP of various bulk density characteristics are disclosed, for example, in U.S. Pat. Nos. 3,030,180 issued Apr. 17, 1962 to Bigot; 3,233,967 issued Feb. 8, 1966 to Shen; 3,684,436 issued Aug. 15, 1972 to Dear et al. and 3,761,573 issued Sept. 25, 1973 to Hinz et al. The latter mentioned Hinz et al. patent does disclose a technique whereby an alkali metal orthophosphate solution is sprayed onto granulated alkali metal tripolyphosphate particles, however, the technique is directed toward the production of high density STPP.

In accordance with the present invention, there is provided a process for preparing granular STPP having a controlled medium range of bulk density which comprises spraying onto rotating finely divided STPP particles having a size distribution of at least 50% by weight -100 mesh, the balance comprising 0-25% -50 + 100 mesh and 0- 25% +50 mesh, an aqueous spray solution of sodium orthophosphate having an Na to P molar ratio of 1.67 to 1 and having a concentration of from about 20 to 65% by weight dissolved phosphate solids until the total amount of dissolved solids sprayed is between about 8 and 15% by weight based on the total weight of STPP to be produced, to form an agglomerated product, and calcining the agglomerated product at a temperature of from 300° to 600°C. to produce granular STPP having a bulk density of about 0.50 to 0.80 g./cc. and a particle size distribution of -20 + 100 mesh (U.S. Standard).

The process of the present invention may be used in connection with rotary dried or sprayed dry finely divided STPP particles and either Form I (Phase I) or Form II (Phase II) STPP product or mixtures thereof within the desired bulk density range may be produced depending upon the temperature and extent of calcination.

The STPP feed material which is subjected to agglomeration and calcining in accordance with the present invention is finely divided STPP which contains substantial proportions of -100 mesh (U.S. Standard) particles. The feed material should contain at least 50% by weight -100 mesh, 0 to 25% by weight -50 +100 mesh and 0 to 25% of particles having the size greater than 50 mesh. Preferably, the finely divided feed material will contain at least about 80% of -100 mesh particles, better still 90 to 100% -100 mesh particles. Screen undersize particles from a conventional STPP spray drying process comprise a particularly suitable starting material. Feed material having these particles size distributions will be sufficient to produce significant yields of a granular product which, after calcination and screening, has a particle size -20 +100 mesh.

Agglomeration is carried out by first providing a suitable rotary kiln agglomerator or dryer apparatus and setting the finely divided STPP particles into rotary motion and spraying the particles with a hot aqueous solution of sodium orthophosphate. This solution is maintained at a temperature of from about 60° to 120°, preferably about 90° to 110°C. The orthophosphate solution will have a Na to P molar ratio of 1.67 to 1 which is stoichiometrically required to produce sodium tripolyphosphate upon calcination.

A preferred embodiment resides in carrying out the spray agglomeration in a rotary calciner which is maintained at calcination temperatures, that is, about 300° to 600°C. In this preferred technique calcination of the agglomerated feed material takes place immediately and there is no need for any intermediate drying steps. In this preferred embodiment, the agglomeration and calcination is essentially a one step continuous process.

A particularly preferred embodiment resides in maintaining the calciner at a temperature of between about 410° and 460°C, as measured by the exit gas temperature, in order to produce finished STPP granules which contain about 15 to 45% by weight Phase I STPP.

It has been found that the concentration of the orthophosphate spray solution should be at least about 20% by weight in order to prepare an agglomerated feed which upon calcination will have a bulk density within the desired range. Concentrations between about 20 to 65% by weight orthophosphate solids are generally suitable with the preferred range being about 30 to 60% by weight. When the process of the present invention is carried out in its preferred form, that is, by spraying the orthophosphate solution onto finely divided STPP particles in a rotary calciner which is being maintained at calcining temperatures, it is preferred that the orthophosphate spray solution have a concentration of about 45 to 60% and that about 9 to 12% of dissolved solids be sprayed. This spray solution is preferably maintained at a temperature of about 90°C to 110°C at the point of introduction into the rotary calciner.

The total amount of solution solids added during the spray agglomeration has been found to be the significant factor in controlling final bulk density. Regardless of concentration, the total amount of spray solids added should be between about 8 and 15% by weight of orthophosphate solids (about 7.3 to 13.8%, expressed as equivalent weight of STPP) based upon the weight of the total STPP produced. Preferably, at least 9% by weight of solids will be employed.

The final product will have a medium range bulk density generally from about 0.50 to 0.80 g./cc. Preferably the final product will have a bulk density between about 0.55 g./cc. and 0.75 g./cc. Products having a bulk density of 0.70 g./cc. ± 0.05 g./cc., better still, ± 0.02 g./cc., are considered most desirable.

The granular STPP product in accordance with the present invention is also characterized as having an improved particle strength as measured by a relatively low degree of frangibility. Frangibility as described and reported herein is measured according to a test described in U.S. Pat. No. 3,337,468 issued to Metcalf et al. on Aug. 20, 1967. According to this test, 100 g. of +100 mesh material is placed on a U.S. Standard 100 mesh screen with three pure gum rubber balls 1 3/8 inch in diameter and shaken 15 minutes with a Ro-Tap sieve shaker. Percent frangibility is a measure of the quantity of particles which pass through a 100 mesh screen. For medium density 0.50 to 0.80 g./cc. particles, a frangibility value below about 25% by weight is generally required for commercial use.

The finished agglomerated and calcined particles are screened and separated so as to isolate granules having a particle size -20 +100 mesh, oversized particles are regranulated and combined into the -20 +100 mesh yield. A total granular yield of at least about 65% by weight -20 +100 mesh particles within the desired bulk density range is generally obtained. Granular yields in the order of 80 to 90% have also been obtained.

The invention is further illustrated by the following examples which are not to be considered as limitative of its scope.

EXAMPLE 1

Finely divided Phase II STPP (a low temperature crystalline modification) was prepared by a conventional rotary calciner process. Phosphoric acid, soda ash and water were reacted in sufficient quantity to give a sodium phosphate solution having a molar ratio of Na to P of 1.67/1 and density of 55° Baume at 100°C. On the finished STPP content basis, this orthophosphate solution has a concentration of about 53% by weight. The solution was evaporated to dryness and the product further calcined to about 400°C in a rotary calciner. The hot product from the calciner was cooled, crushed and milled into essentially -100 mesh (U.S. Standard) powder. About 10,000 g. of the -100 mesh Phase II product transferred into an 18 × 18 inch horizontal drum-agglomerator, equipped with small lift flights. The drum was rotated at 15 rpm. The powder was agglomerated by spraying onto it an orthophosphate solution having an Na/P molar ratio of 1.67/1 which was maintained at a temperature of 90°C. The orthophosphate spray solution had a concentration of 32.7% by weight, or 30% by weight in terms of equivalent weight of STPP. The total amount of solution sprayed was 34.4% by weight based on the weight of total STPP produced. The total amount of solids sprayed amounted to 14.3% by weight of orthophosphate based on the total weight of STPP produced. The spraying was done with an air atomized spray nozzle. At the end of the test, the sprayed product was kept in the rotary drum-agglomerator and calcined at a temperature of about 350°C to convert it into anhydrous Phase II STPP. The calcining was done with internal heat supplied by a 1.5 inch diameter burner. The hot product was removed from the calciner, cooled in stainless steel trays and screened to separate the -20 +100 mesh granular fraction. The +20 mesh "oversized" fraction was crushed in a laboratory granular mill and the -20 +100 mesh portion was included in the total granular yield. Yield was 80% by weight, based on the total equivalent weight of STPP processed from both spray solution and feed particles of -20+100 mesh granules having a bulk density of 0.65 g./cc. and a frangibility of 22% by weight.

EXAMPLE 2

Example 1 was repeated except that a spray solution concentration of 38.2% by weight orthophosphate solids was employed. The quantity of solution sprayed was 29.9% and the quantity of solids sprayed was 13.8%. Granular yield was 90% by weight of -20+100 mesh granules having a bulk density of 0.70 g./cc. and a frangibility of 11%.

EXAMPLE 3

Finely divided STPP feed material was prepared as described in EXAMPLE 1 except that Phase I was prepared by heating the product to between 480° and 530° in the rotary calciner before being cooled, crushed and milled into essentially -100 mesh particles. As in Example 1 about 10,000 grams of the essentially -100 mesh powder was agglomerated using a spray solution having the concentration of 32.7% by weight orthophosphate solids. The total amount of solution sprayed was 31% and the total quantity of solids sprayed was 12.3% by weight of orthophosphate solids based on STPP. The agglomerated products were further calcined, screened and separated as in Example 1. Granular yield was 65% of -20+100 mesh granules having a bulk density of 0.55 grams/cc. and a frangibility of 22%.

EXAMPLE 4

Example 3 was repeated except there was employed a spray solution having a concentration of 38.2%, the total amount of solution sprayed amounted to 31.8% and the total amount of solids sprayed amounted to 14.8%. Granular yield was 85% of -20+100 mesh granules having a bulk density of 0.66 g./cc. and a frangibility of 7%.

EXAMPLE 5

Another finely divided Phase I STPP feed was obtained by using screen undersize from a conventional rotary dryer process. The rotary dryer process was the same as described in Example 1, but the hot STPP discharge from the rotary calciner was cooled, crushed and then screened to separate the -20+100 mesh high density granular fraction. After removing the oversize and granular fractions, the screen undersized product of this operation contained 7% by weight of -20+80 mesh, 8% -80+100 mesh, and 85% -100 mesh particles. Two 10,000 gram portions of this finely divided feed were agglomerated using the same rotary drum-agglomerator as in Example 1. In the first run, the spray solution concentration was 27.2% by weight orthophosphate solids, the amount of solution sprayed was 30.0% based on STPP and the quantity of solids sprayed was 10.4% by weight. Agglomerate product was calcined, cooled and screened as in Example 1. In the first run, the granular yield was 80% of -20+100 mesh granules having a bulk density of 0.73 g./cc. and a frangibility of 6%. In the second run, the solution concentration was the same but the quantity of solution sprayed was 37.2%, the total amount of solids sprayed was 13.1%. The granular yield in the second run was 90% of -20+100 mesh particles having a bulk density of 0.71 g./cc. and a frangibility value of 3%.

EXAMPLE 6

Another finely divided Phase I STPP was obtained by using screen undersize particles from a commercial spray drying process. The screen undersize feed material contained about 40% by weight -50+100 mesh and 60% -100 mesh particles. This screen undersize was fed continuously into an internally heated 30 feet long and 6.5 feet diameter rotary calciner. Heating was effected with a natural gas flame at the inlet of the calciner. The calciner had an exit gas temperature of about 425°C. Agglomeration was carried out by spraying onto the rotating STPP particles, a orthophosphate feed solution containing 57.8% by weight dissolved solids (53% by weight expressed as equivalent weight of STPP). The quantity of solution sprayed was 21% by weight and the total amount of solids sprayed was 11.7% by weight (10.7% by weight expressed as equivalent weight of anhydrous STPP). After calcining the product was screened and separated and as in Example 1 to isolate -20+100 mesh granules. Granular yield was 65% by weight of product having a bulk density of 0.71 g./cc. and a frangibility value of 3%. The product contained about 25% by weight Phase I STPP.

EXAMPLE 7

Example 6 was repeated except that the feed material had a particle size distribution of 20% by weight +50 mesh 30% by weight -50+100 mesh and 50% by weight -100 mesh. Granular yield was 57% by weight and bulk density was 0.68 g./cc. 

What is claimed is:
 1. A method of preparing granular sodium tripolyphosphate of medium density which comprises spraying onto rotating finely divided sodium tripolyphosphate particles having a size distribution of at least 50% by weight -100 mesh, 0 to 25% by weight -50+100 mesh and 0 to 25% by weight +50 mesh, an aqueous spray solution of sodium orthophosphate having a concentration of from about 20 to 65% by weight until the total amount of dissolved solids sprayed is between 8 and 15% by weight based on the total weight of sodium tripolyphosphate to be produced, to form an agglomerated product and calcining the agglomerated product at a temperature of 300° to 600°C to produce granular sodium tripolyphosphate having a bulk density between about 0.50 and 0.80 grams per cubic centimeter and a particle size of -20+100 mesh.
 2. The method of claim 1 wherein the finely divided sodium tripolyphosphate particles contain at least 80% by weight -100 mesh particles.
 3. The method of claim 1 wherein the total amount of solids sprayed is at least about 9% by weight.
 4. The method of claim 1 wherein the granular sodium tripolyphosphate produced has a bulk density between 0.55 and 0.75 grams per cubic centimeter.
 5. The method of claim 1 wherein the process is carried out in a rotary calciner maintained at calcination temperatures of about 300° to 600°C.
 6. The method of claim 5 wherein the spray solution has a concentration of between about 45 and 60% by weight.
 7. The method of claim 5 wherein the total amount of solids sprayed is between about 9 and 12% by weight.
 8. The method of claim 5 wherein the bulk density of the sodium tripolyphosphate granules produced is between about 0.55 and 0.75 grams per cubic centimeter.
 9. The method of claim 5 wherein the finely divided sodium tripolyphosphate feed particles are composed of screen undersized particles from a spray dryer.
 10. The method of claim 5 wherein the calciner is maintained at a temperature between 410° to 460°C. and the final product contains between about 15 and 45% by weight Phase I sodium tripolyphosphate.
 11. The method of claim 5 wherein the aqueous spray solution of sodium orthophosphate is introduced at a temperature between about 90° and 110°C. 